Fixed door-curtains in vehicles

Without this fix it was impossible to have a door closed but the curtain open, which kind of defeats the purpose of using a curtain instead of an opaque door in the first place.

Reversed order of checking in vehicle::next_part_to_open and
vehicle::next_part_to_close, as previously you would close a curtain
before a door and open a door before a curtain.

I suspect when the feature had been originally tested there was a
special z level for on_windshield that put them farther outside than a
door, but currently they are at the default z level which is farther
inside than a door.